前言
关于 CSS 的介绍，基本上告一段落了。在该博客中将介绍如何通过 CSS 去设置一个 HTML 元素，显示在 Web 页面的位置。定位
四种定位方式比较
| Static | 是 | 是 | 否 |
| Fixed | 否 | 否 | 是 |
| Relative | 是 | 是 | 否 |
| Absolute | 否 | 否 | 是 |
注意
通过观察表格，大家可以看出：在四中定位方式中，凡是与文档流有关的都会占据文档流空间，且不可重叠；凡是与文档流无关的都不会占据文档流空间，且可以重叠。原因是什么呢？HTML 文档流的布局是流式布局（fluid layout），像流水一样布局，怎么可以重叠，没有空间又怎么能布局元素！
看了上边的描述，你肯定还是一头雾水，不知道这四种定位方式到底说的是什么，在什么时候该用什么方式的定位。在定位一个元素的时候，我们通常需要选择一种定位方式来确定一个元素的布局方式，还需要设置的它的偏移量，那么问题就来了，偏移量的偏移基准是什么？
代码
<!DOCTYPE html><html><head><style>body { background-color: lightgray;}p.pos_fixed{ position:fixed; background-color: red;}</style></head><body><p class="pos_fixed">This is a paragrap.</p></body></html>效果
观察发现，元素的左方和上方，会有一定的边距，这是为什么？“这是浏览器为 body 添加的默认边距”，这的这么简单吗？我们来试试。代码
// 改变 body 的样式body { background-color: lightgray; margin: 0px;}效果
观察发现，元素左边的边距确实没有了，但是上方的边距还在。“这是浏览器为 body 添加的默认填充”，那么我可以明确的告诉你，这不是答案。那么答案到底是什么？
代码
// 改变 .pos_fixed 类的样式p.pos_fixed{ position:fixed; background-color: red; margin: 0px;}效果
我们惊奇的发现，我们的问题解决了。原来是这样：“浏览器为 p 默认添加了上边距（在 p 前后分别添加一个换行）”，但是对于 Fixed 定位方式，我们还需要注意一点，我们来看看如下效果
代码
// 改变样式表p { margin: 0px;}p.pos_fixed{ position:fixed; background-color: red;}// 改变内容<p>This is another paragrap</p><p>This is another paragrap</p><p>This is another paragrap</p><p>This is another paragrap</p><p class="pos_fixed">This is a paragrap.</p><p>This is another paragrap</p><p>This is another paragrap</p><p>This is another paragrap</p><p>This is another paragrap</p>效果
我们发现，Fixed 定位的元素，确实没有占据了文档流的空间，当滚动内容是，位置也没有变化。但是默认情况下，Fixed 定位的元素的 top 还是会受到文档流的影响的。这与表格中的描述有矛盾，还请大家注意，非常愿意与大家一起探讨原因之所在。
代码
<!DOCTYPE html><html><head><style>body { background-color: lightgray; margin: 0px;}div { width: 100px; height: 60px;}#div1 { background-color: red; top: 10px;}#div2 { background-color: blue;}</style></head><body><div id="div1"></div><div id="div2"></div></body></html>效果
观察发现，div1 和 div2 按照流式布局，在没有设置定位方式的情况下，设置了 top 是没有任何效果的。原因想必大家都知道，我们来试试设置定位方式后的效果
代码
// 改变 div2 的定位方式，且设置偏移量#div2 { background-color: blue; position: relative; top: -10px; left: 10px;}效果
观察发现，设置定位方式为 relative 后，偏移量产生了效果，相对于正常位置向右向上分别便宜了 10px，且元素可以重叠在一起。我们接下来看看，如何通过 z-index 来控制元素的重叠。
代码
// 改变 div2 的 z-index 属性#div2 { background-color: blue; position: relative; top: -10px; left: 10px; z-index: -1;}效果
改变 div2 的 z-index 后，div2 显示在了 div1 的后边。关于 z-index，我们需要了解以下几点：默认值为 0；只作用于 relative 定位的元素；值大的元素排在前面。
代码
<!DOCTYPE html><html><head><style>body { background-color: lightgray; margin: 20px 20px;}#strap { background-color: green; width: 200px; height: 120px;}#div1 { background-color: red; width: 160px; height: 90px;}#div11 { background-color: blue; width: 100px; height: 40px;}#div12 { background-color: orange; width: 100px; height: 40px;}</style></head><body><div id="strap"> <div id="div1"> <div id="div11"></div> <div id="div12"></div> </div></div></body></html>效果
在未设置定位和，偏移量的情况下，效果如图。我们来设置绝对定位试试。
代码
// 设置 div12 的定位方式为 absolute#div12 { background-color: orange; width: 100px; height: 40px; position: absolute;}效果
观察后，我们发现没有任何效果。若只设置 absolute 定位方式，不设置偏移量，将不会影响元素的布局。接下来我们来设置偏移量。代码
// 设置 div12 的偏移量#div12 { background-color: orange; width: 100px; height: 40px; position: absolute; top: 0px; left: 0px;}效果
观察发现，div12 脱离了文档流，移动到了窗口的左上角，而我们在这里设置了 body 的上边距和左边距。接下来我们来设置 div12 祖先元素的定位。
只有一个定位的父元素
// 设置组向 strap 的定位方式#strap { background-color: green; width: 200px; height: 120px; position: relative;}#div12 { background-color: orange; width: 100px; height: 40px; position: absolute; right: 10px; bottom: 10px;}多个定位的父元素
// 设置 div1 的定位方式#div1 { background-color: red; width: 160px; height: 90px; position: relative;}只有一个定位的父元素
多个定位的父元素
本来想以这篇博客，来结束 CSS 的基础介绍。但是已经写了这么多，还是有很多知识点没有涉及到，我本不是一个拖拖拉拉的人。那么，在接下来的博客，我会继续介绍 CSS 的基础知识。“您好，您的文章知识点丰富，但是从读者的角度想可读性略差，您精通css建议您考虑将页面多少美观，整洁一些，以更好达到分享的目的。” ，这样的消息不知你是否收到过，我是收到过。那么在介绍完 CSS 基础知识后，如果有机会的话，我会详细说说如何来定制自己的博客页面。另外，由于最近在赶一个项目，不常写博客。今天突然发现，有几个消息和评论没有及时的回复，在此我表示抱歉，以后定会在一天内给予回复。你也可以加我的 QQ：1829871867，方便及时的联系我。
What are self-closing tags? Give an example.Apr 27, 2025 am 12:04 AMSelf-closingtagsinHTMLandXMLaretagsthatclosethemselveswithoutneedingaseparateclosingtag,simplifyingmarkupstructureandenhancingcodingefficiency.1)TheyareessentialinXMLforelementswithoutcontent,ensuringwell-formeddocuments.2)InHTML5,usageisflexiblebutr
Beyond HTML: Essential Technologies for Web DevelopmentApr 26, 2025 am 12:04 AMTo build a website with powerful functions and good user experience, HTML alone is not enough. The following technology is also required: JavaScript gives web page dynamic and interactiveness, and real-time changes are achieved by operating DOM. CSS is responsible for the style and layout of the web page to improve aesthetics and user experience. Modern frameworks and libraries such as React, Vue.js and Angular improve development efficiency and code organization structure.
What are boolean attributes in HTML? Give some examples.Apr 25, 2025 am 12:01 AMBoolean attributes are special attributes in HTML that are activated without a value. 1. The Boolean attribute controls the behavior of the element by whether it exists or not, such as disabled disable the input box. 2.Their working principle is to change element behavior according to the existence of attributes when the browser parses. 3. The basic usage is to directly add attributes, and the advanced usage can be dynamically controlled through JavaScript. 4. Common mistakes are mistakenly thinking that values need to be set, and the correct writing method should be concise. 5. The best practice is to keep the code concise and use Boolean properties reasonably to optimize web page performance and user experience.
How can you validate your HTML code?Apr 24, 2025 am 12:04 AMHTML code can be cleaner with online validators, integrated tools and automated processes. 1) Use W3CMarkupValidationService to verify HTML code online. 2) Install and configure HTMLHint extension in VisualStudioCode for real-time verification. 3) Use HTMLTidy to automatically verify and clean HTML files in the construction process.
HTML vs. CSS and JavaScript: Comparing Web TechnologiesApr 23, 2025 am 12:05 AMHTML, CSS and JavaScript are the core technologies for building modern web pages: 1. HTML defines the web page structure, 2. CSS is responsible for the appearance of the web page, 3. JavaScript provides web page dynamics and interactivity, and they work together to create a website with a good user experience.
HTML as a Markup Language: Its Function and PurposeApr 22, 2025 am 12:02 AMThe function of HTML is to define the structure and content of a web page, and its purpose is to provide a standardized way to display information. 1) HTML organizes various parts of the web page through tags and attributes, such as titles and paragraphs. 2) It supports the separation of content and performance and improves maintenance efficiency. 3) HTML is extensible, allowing custom tags to enhance SEO.
The Future of HTML, CSS, and JavaScript: Web Development TrendsApr 19, 2025 am 12:02 AMThe future trends of HTML are semantics and web components, the future trends of CSS are CSS-in-JS and CSSHoudini, and the future trends of JavaScript are WebAssembly and Serverless. 1. HTML semantics improve accessibility and SEO effects, and Web components improve development efficiency, but attention should be paid to browser compatibility. 2. CSS-in-JS enhances style management flexibility but may increase file size. CSSHoudini allows direct operation of CSS rendering. 3.WebAssembly optimizes browser application performance but has a steep learning curve, and Serverless simplifies development but requires optimization of cold start problems.
HTML: The Structure, CSS: The Style, JavaScript: The BehaviorApr 18, 2025 am 12:09 AMThe roles of HTML, CSS and JavaScript in web development are: 1. HTML defines the web page structure, 2. CSS controls the web page style, and 3. JavaScript adds dynamic behavior. Together, they build the framework, aesthetics and interactivity of modern websites.
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
Hot Tools
Atom editor mac version download
The most popular open source editor
SecLists
SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.
Dreamweaver CS6
Visual web development tools
SublimeText3 Chinese version
Chinese version, very easy to use
DVWA
Damn Vulnerable Web App (DVWA) is a PHP/MySQL web application that is very vulnerable. Its main goals are to be an aid for security professionals to test their skills and tools in a legal environment, to help web developers better understand the process of securing web applications, and to help teachers/students teach/learn in a classroom environment Web application security. The goal of DVWA is to practice some of the most common web vulnerabilities through a simple and straightforward interface, with varying degrees of difficulty. Please note that this software
